A carbon fiber has excellent properties such as high specific strength and heat resistance and is used as one of a reinforcing material of a composite material. However, since the carbon fiber has poor hydrophilicity when compared with other reinforcing materials, there has been made a study on the improvement of functionality required as a composite material by performing a treatment of a carbon fiber so as to enhance affinity and adhesion between the carbon fiber and an organic material such as a resin or an inorganic material, which serves as a matrix (see, for example, Patent Literature 1).
Also, there has recently been made a study on the prevention of a decrease in a melting temperature and the improvement of dispersibility by treating a metal ultrafine powder, etc. and the surface thereof.
A conventional oxidation treatment method of a carbon fiber and a metal fine powder includes, for example, a wet oxidation method using an aqueous solution of acids, ozonized water and the like, and a dry oxidation method using air, plasma, an ozone gas and the like, and ozone has been well known as a very powerful oxidizing agent since long ago.
There has sufficiently been made a study on the wet oxidation method using ozonized water since long ago. For example, Patent Literature 2, Patent Literature 3 and the like are exemplified.
In contrast, the dry oxidation method requires neither a post-treatment such as neutralization, nor a treatment of a large amount of a waste liquid generated during neutralization and rinsing, and is therefore advantageous from the viewpoint of cost. Thus, the dry oxidation method has recently become of major interest as a method to which, a continuous mass treatment is expected. One of this dry oxidation method includes, for example, a treatment method using an oxidizing gas such as ozone. For example, Patent Literature 4, Patent Literature 5 and the like are exemplified.